This invention relates to methods for cleaning articles. More particularly, this invention relates to methods for cleaning debris from internal channels of articles such as, for example, gas turbine engine components. This invention also relates to apparatus used to clean the internal channels of such articles.
In a typical gas turbine engine, compressed air is mixed with fuel in a combustor and ignited, generating a flow of hot combustion gases through one or more turbine stages that extract energy from the gas, producing output power. Each turbine stage includes a stator nozzle having vanes that direct the combustion gases against a corresponding row of turbine blades extending radially outwardly from a supporting rotor disk. The vanes and blades are subject to substantial heat load, and, because the efficiency of a gas turbine engine is related to gas temperature, the continuous demand for efficiency translates to a demand for airfoils that are capable of withstanding higher temperatures for longer service times.
Gas turbine airfoils on such components as vanes and blades are usually made of superalloys and often employ internal cooling channels to avoid overheating the component to temperatures beyond the capabilities of these materials. The term “superalloy” is usually intended to embrace iron-, cobalt-, or nickel-based alloys, which include one or more other elements including such non-limiting examples as aluminum, tungsten, molybdenum, titanium, and iron. The internal air-cooling of turbine airfoils is often accomplished via a complex cooling scheme in which cooling air flows through channels, often serpentine in shape, within the airfoil (“internal channels” or “internal cooling channels”) and is then discharged through a configuration of small cooling holes at the airfoil surface. Convection cooling occurs within the airfoil from heat transfer to the cooling air as it flows through the internal cooling channels.
A considerable amount of cooling air is often required to sufficiently lower the surface temperature of an airfoil. This cooling air generally contains particulate matter, such as dust, sand, mineral deposits, and other foreign matter entrained in the air taken in to cool the engine. The particles can adhere to the walls of the internal cooling channels and accumulate to a point where the cooling air flow through the channel is partially or completely restricted. The resulting restrictions in cooling airflow promotes higher component operating temperatures and the accompanying risk of performance problems, including severe damage to the component due to overheating.
In order to extend the life of costly gas turbine engine components, debris accumulations in the internal cooling channels are periodically removed by any of various cleaning processes, including autoclave processes wherein the component is exposed to high temperature and high pressure fluid for a period of time; and ultrasonic cleaning, wherein the component is immersed into a cleaning fluid and ultrasonically agitated. Both of these methods are effective in cleaning simple components, but are not nearly as effective for cleaning components with complicated internal passages, for example, as found in a gas turbine blade. Effective cleaning processes remove substantially all of the debris accumulated within the internal channels; at the same time, cleaning processes strive for efficiency, due to the large numbers of components, such as airfoils, that must be cleaned when overhauling even a single gas turbine engine. Therefore, there is a need to provide an effective methods and apparatus for efficiently cleaning gas turbine components, especially those with complicated geometry, as in the exemplary case of a gas turbine blade with internal cooling passages.